The reaction of silicon and nitrogen is highly exothermic, thus, when a nitrogen-rich atmosphere is caused to react on silicon under conditions of a controlled linear rise in temperature, the reaction runs away after a threshold temperature has been reached, thereby exceeding the melting point of silicon and producing a part having residual free silicon and large pores, with an apparent density which is much less than the density of silicon nitride obtained after completely nitriding a preformed part made of silicon. Analogous phenomena are observed whenever there is an exothermic reaction between a metal in the solid state in the form of a preformed part or a powder, and a gas such as nitrogen or oxygen.
Attempts have been made to mitigate this difficulty by performing the reaction at successive temperature stages, and monitoring each stage so that its temperature is not increased until reaction equilibrium appears to have been reached. Complete nitriding can be obtained by such a method at the cost of an excessive length of time.
Proposals have also been made to keep the pressure of the nitrogen in contact with the silicon to be nitrided at a predetermined level, and, once the reaction has started, to keep the temperature at a predetermined level until no further drop in nitrogen pressure is observed over some minimum duration, which indicates that the reaction has reached equilibrium. The temperature is then raised to a higher level and the operation is repeated in successive stages up to a temperature of about 1400.degree. C., which is a little below the melting point of silicon. Although complete nitriding can thus be obtained, and the resulting parts have a specific density close to the theoretical maximum value, the nitriding operation remains lengthy.
Preferred implementations of the present invention thus provide a method of manufacturing parts or powders made of a compound of silicon or of a metal by exothermic reaction of the parts or the powder of silicon or of a metal in the solid state with nitrogen or with a nitrogen-rich gas, said method being rapid, but avoiding reaction runaway, and at least in the initial phases of the reaction avoiding any melting of the silicon or of the metal, which would otherwise lead to porosity in the part to be manufactured. The invention also seeks to provide a method which can be adapted to nitriding large parts as well as small parts.